The broad objective is to develop a cell culture system permitting relatively long-term study of skeletal muscle fiber growth and experimentally induced chronic relaxation and to relate chronic relaxation to the ensuing fiber atrophy and the underlying biochemical changes responsible for the progressive loss of myofibrillar proteins. Specifically, we have already developed methods for long-term culture of chick embryo muscle fibers, for the analysis of muscle specific protein synthesis and accumulation, and for the synthesis and activity of specific messenger RNA's for particular muscle cell polypeptides. We now plan an extensive series of experiments designed to apply these methods under circumstances designed to block normal spontaneous contractions seen in cultured muscle fibers and which produce thereby a condition of chronic and reversible relaxation. Under these conditions skeletal muscle fibers cease maturation and/or begin a process of atrophy which may be reversed upon removal of the relaxing agent. We want to study a limited number of known muscle relaxants, mostly benzodiazepines, for their ability to induce direct relaxation of muscle fibers since we already have evidence that at least one of these compounds, diazepam, is active in this respect. Following our ability to induce a reversible hypotrophic response in muscle fibers, we will examine the effect of chronic inactivity on: (1) myosin synthesis, (2) myosin accumulation, (3) myosin degradation and turnover, (4) whether and to what extent relaxation or inactivity-induced atrophy interferes with a normal sequence of specific gene transcription (for myosin heavy chains) and processing of mRNA to the point of normal translation. Reversibility studies may provide as well some insights into those processes required for establishing normal muscle fiber maintenance during recovery from muscle wasting.